Read/write heads, such as that used in disk drives, fly close to the surface of a magnetic or magneto-optical disk and read or write data on the surface of the disk. The closer the read/write head flies to the surface of the disk, the more information that may be written on the disk, i.e., the information density is increased. Currently, the typical separation between a read/write head and the surface of the disk is approximately 1.mu." (microinch). While it is desirable for a read/write head to fly as close as possible to the surface of the disk, it is important that the read/write head does not contact the disk or defects on the disk. A defect on the surface of the disk that physically contacts the read/write head may damage the read/write head, the disk, or both. Thus, care must be taken to assure that there are no defects on the surface of the disk that are greater than the fly height of the read/write head such that the defect may contact the read/write head.
Glide heads are used to test the surface of disks for defects that are large enough to contact a read/write head during use. FIGS. 1A and 1B illustrate a respective bottom plan view and side view of a conventional glide head 2. Glide head 2 has a leading end 8 and a trailing end 10. Two side rails 4 and 6 extend from the leading end 8 to the trailing end 10. Glide head 2 includes taper 12 at the leading end 8 of side rails 4 and 6. Taper 12 permits air to be forced under air bearing surfaces 14 and 16 to provide lift to glide head 2 as it flies over a disk. A transducer 19, such as a piezoelectric crystal, is mounted on the top surface 18 of glide head 2 and is used to convert energy from an impact between the glide head 2 and a defect on a disk being tested into an electrical signal.
FIG. 2 illustrates glide head 2 mounted on a suspension arm 20 and flying over the surface 24 of a rotating disk 22. Disk 22 rotates in the direction of arrow 25 about a spindle 26. A linear actuator (not shown) controls the radial position of glide head 2 with respect to disk 22 by moving suspension arm 20 as illustrated by arrow 21.
It should be understood that typically, a top surface 24A and a bottom surface 24B of disk 22 is tested at the same time by a downward facing glide head 2A and an upward facing glide head 2B, respectively, as shown in the side view illustrated in FIG. 3. Glide heads 2A and 2B are mounted on respective suspension arms 20A and 20B, which are controlled by linear actuator 28.
During a test, disk 22 rotates to produce a high linear velocity between disk 22 and glide head 2. The high linear velocity drives air between the surface 24 of disk 22 and glide head 2, which produces lift on air bearing surfaces 14 and 16 of glide head 2. Thus, glide head 2 is said to "fly" over surface 24 of disk 22. As disk 22 rotates, glide head 2 is moved laterally over a radius of disk 22 by linear actuator 28 (shown in FIG. 3). The lateral movement of glide head 2 is slow relative to the rotation of disk 22.
Glide head 2 detects a defect on surface 24 of disk 22 by physically contacting the defect. When glide head 2 impacts a defect, mechanical energy is generated in the form of a vibration. The mechanical energy is transferred through glide head 2 and is received by a transducer 19 that is mounted on glide head 2. The transducer converts the mechanical energy into an electric signal, which can be measured by the glide testing apparatus to determine the size of the defect.
The magnitude of the electric signal generated by transducer 19 is determined by the strength of the impact between glide head 2 and the defect, i.e., the greater the impact, the more mechanical energy generated and the greater the magnitude of the electric signal produced by transducer 19. Consequently, by holding glide head 2 at a constant fly height over surface 24 of disk 22, the relative size of defects that are encountered can be determined by examining the magnitude of the electric signal produced by transducer 19. When a defect is smaller than the fly height of the glide head 2, the defect will not be detected. Thus, the fly height of the glide head 2, i.e., the distance between the surface 24 of disk 22 and the lowest flying point on glide head 2, should be no greater than the height of the smallest defect to be detected, which is the same as the desired fly height of the read/write head to be used with disk 22.
During use, glide head 2 vibrates even when it does not strike a defect. This causes transducer 19 to produce an output voltage noise signal.
It is important for glide head 2 to maintain a constant fly height. To maintain a constant fly height, the rotation or angular velocity of disk 22 is automatically adjusted as glide head 2 moves laterally across the radius of disk 22. In other words, disk 22 will automatically rotate faster when glide head 2 is at an inside diameter than when glide head 2 is at an outside diameter. Thus, as glide head 2 moves across disk 22, glide head 2 will encounter a constant linear velocity, which will provide a constant fly height. Examples of glide testers that strictly control the fly height of glide heads over disks are model numbers MC 950 and MG 250 by Phase Metrics Inc., and model number 6800-50D produced by Hitachi, Inc.
Prior to being used to test magnetic or magneto-optical disks for defects, each glide head is individually calibrated to ensure that it will fly at the desired fly height, i.e., no greater than the height of the smallest defect to be detected. Calibration of the fly height of the glide head is typically performed on a dedicated fly height tester, such as the Phase Metrics DFHT manufactured by Phase Metrics, Inc. located in Fremont, Calif. Calibration of the fly height of a glide head determines the appropriate linear velocity to use with that particular glide head to produce the desired fly height. The glide testing apparatus then uses that appropriate linear velocity with that particular glide head.
In addition, the output signal of the glide head is calibrated to ensure that an unacceptably large defect will properly register. Calibration of the output signal may be accomplished with a dedicated calibration disk that has calibration bumps of a known size. When the glide head contacts one of the calibration bumps, the output signal can be adjusted so that it will be detectable during testing. Thus, for a defect of a specific size the glide head is calibrated to produce an output signal of a specific magnitude. By way of an example, glide head 2 may be calibrated to produce an output signal of 4 volts when glide head 2 contacts a 1.mu." defect.
Unfortunately, conventional glide heads wear out from testing disks for defects. With use, a conventional glide head will produce progressively smaller output signals when detecting defects of the same size. Consequently, while a glide head may be calibrated initially to produce a large output signal when detecting a defect, eventually the output signal of the glide head will be too small to register or below the background noise of the system when detecting a defect of the same size. It has been found that a conventional glide head has a useful life of approximately fifty disks before the output signal becomes lost in the background noise.
Once the useful life of a conventional glide head is over, that glide head must be replaced. The worn out glide head is no longer useful and is discarded. Replacing a glide head requires calibration and installation of a new glide head. Consequently, a significant cost, including time and labor, is associated with discarding worn glide heads and calibrating and installing new glide heads.
Thus, there is a need for a glide head that has a longer useful life.